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Optical Manufacturing Guidelines for Medical OEMs

Wednesday, 01 February 2012

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For those manufacturers integrating
individual optical components, another
consideration when selecting optics is
the focal length of the optical system —
the ratio of the focal length of the tube
lens to the focal length of the objective
lens. To change the total magnification
of an optical system, it is often easiest to
change the objective lens since there is
typically more selection from manufacturers;
in cases where there is a specific
objective lens performance characteristic
that must be preserved, the tube lens
may be changed.

Depending on the system, the illumination
source may be a laser, LED, or
standard halogen or mercury bulb. But
even the selection of an illumination
source sometimes can affect the components
specified. Both the mechanical
interface of the entire optical system and
its total demands are important factors,
even when they do not appear to directly
affect performance. Every optical component
and detector is rated to withstand
a certain amount of energy; integrators
working with high-power energy sources
such as lasers should check with the
manufacturer of all components for
compatibility. In addition, many samples
are temperature-sensitive. As a result,
the experimental apparatus often is
designed to remove heat generated by
the power or light source. In many cases,
using an external cold-light source with
a fiber-optic connection is an optimal
solution.

Imaging ConsiderationsCameras and other imaging detectors
are another key component of today’s
medical device diagnostic systems whenever
viewing, capturing, or archiving
images is required. In addition, integrators
of the future may have to ensure
that images they collect will be suitable
for uploading to laboratory information
management systems. Some questions to
consider when selecting a suitable digital
imaging device include:

How cool does it need to be? Depending
on the application, some cameras
require cooling. Cooled cameras are
more sensitive at low light levels and
therefore are particularly well suited to
fluorescence imaging, but are usually
also more expensive. Liquid nitrogen,
water, air, or Peltier thermo-electric systems
can be used to cool camera components.
Some of these choices require
additional space or external equipment.

Will it deliver the images I need? Some
people mistakenly believe that more
megapixels mean better-quality images.
The truth is that designers must match
the optical resolution of the microscope
system or spot size projected on
the charge-coupled device (CCD)
detector with the digital resolution of
the camera to avoid over- or under-sampling.
The spot size of the projected
image depends on the objective lens
magnification, numerical aperture,
tube lens, and camera mount magnification.
The resolution of the CCD
varies based on the size and number of
pixels. The desired optical resolution
should be considered first, and then
digital spatial resolution should be
determined, as it is the limiting factor
in achieving the system’s overall resolution.
This phenomenon is explained by
Nyquist’s criterion, which says that twoto-
three pixels on a CCD chip are
required to resolve the smallest feature
the optical system is capable of reproducing.
Using a smaller number of pixels
results in image degradation. A larger
number of pixels does not assist in
resolution and may make it more complex
and costly to share, transmit, and
store images.

What image analysis or other software
capabilities do I need? Digital cameras can
be used either with off-the-shelf or integrator-
designed software. Integrators
typically look at ease of use, efficiency,
the ability to document system parameters,
and the delivery of reliable results
in considering whether and how to
design software. With the growth of
automation, hardware components are
increasingly becoming motorized. In -
tegrators need to take into account the
compatibility of all components in tandem,
in order to develop code that can
optimize the combined capabilities of all
system elements; in addition, the software
must be customized to suit the
needs of life scientists and medical professionals.

Conclusion

With knowledge and careful planning,
system integrators can select optics, filters, light sources, and cameras
for their medical diagnostic instrumentation.
With the requirements of the
FDA becoming more stringent, integrators
need the support of a trusted optical
supplier that adheres to a comprehensive
quality program for the manufacture
of its high-performance optical
components. But while suppliers have a
plethora of data about performance,
not all of that data is in the public
domain. Select a supplier that deeply
understands the business requirements,
work together closely over time, and be
prepared to provide detailed requirement
parameters, which will allow the
supplier to specify whether a given optical
component will meet your needs.
The supplier also needs to understand
logistical needs and should be able to
handle delivery requirements once the
product is on the market. This will
ensure the delivery of products that
meet both the manufacturer’s requirements
and those of its customers.

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